Tamper-resistant hermetically sealed containers are well known in the packaging art. Such hermetically sealed containers can be produced by providing a cap with a cap liner, securing the cap to a container and subjecting the capped container to an induction field thereby sealing a portion of the cap liner across the top of the container. Typically the cap liner in the cap is a laminated structure wherein the laminate layer secured across the top of the container is a metallic foil or ply coated with a heat activated sealant. After the metallic foil has been secured to the container to provide a hermetic seal, the cap is removed from the container and the remainder of the laminated cap liner is retained in the cap for resealing the container after the hermetic seal is broken.
Known cap liners used in producing hermetically sealed containers suffer from a number of disadvantages. Wax laminated cap liners are subject to delamination and thus, part or all of the cap liner which should remain in the cap may fall out of the cap. If the cap liner does delaminate, the cap liner may be rendered ineffective for sealing the container. After removal of the foil seal, delamination of the cap liner can result in contamination of the container contents or can result in physical degradation of the container contents by allowing exposure of the contents to air and moisture due to insufficient resealing of the container by the delaminated cap liner. Failure of the cap liner to remain in the cap is also annoying to consumers of the container's contents, and it is likely that a consumer would merely discard a cap liner which has fallen out of the cap. A cap without a cap liner may not adequately reseal the container.
One example of a cap liner used to produce a hermetically sealed container is a cap liner having a pulpboard backing, a wax coating, aluminum foil and a heat sealable film coating. The pulpboard backing having a wax coating is retained in the cap after sealing of the container. The pulpboard backing often falls out or is pulled out of the cap and thus prevents adequate resealing of the container after the hermetic seal has been broken. If it remains in the cap, the pulpboard backing frequently becomes saturated with product, disintegrates and falls into the container, thereby contaminating the product. Another example of a cap liner useful in producing a hermetically sealed container is disclosed in U.S. Pat. No. 3,928,109. In this patent, the cap liner comprises a metallic-non-metallic-metallic laminate. One of the metallic plys is bonded to a container upon exposure of the cap container to an induction field. Since the cap liner of U.S. Pat. No. 3,928,109 requires three plys, there are usually four bonding layers associated with this cap liner. The relative bonding strengths of these bonding layers must be carefully controlled or delamination of the cap liner will result. Preparation of the multi-layer laminate of U.S. Pat. No. 3,928,109 can be time-consuming and expensive. In addition, the presence of at least three plys in the cap liner laminate provides many potential delamination sites.